1. Field of the Invention
The present invention relates to an optical device that is essentially used in the field of optical communication, and to a method for fabricating it. In particular, the invention relates to an optical device having a specific surface profile structure favorable for transmission gratings, Fresnel lenses, microlens arrays and the like, and to a method for fabricating it.
2. Related Art
With the increase in the capacity of information communication in the field of optical communication, many optical devices such as diffraction gratings, microlens arrays and others have become much used therein. These are tabular optical devices that have a predetermined protrusion-groove microstructure profile on their surfaces for utilizing light diffraction or retraction through or on the profile structure thereof.
Various methods are known for forming such a protrusion-groove profile structure profile on the surfaces of tabular optical devices. One method that is preferred for inexpensive and industrial-scale fabrication of the optical devices of the type is a resin forming technique. This comprises uniformly casting a UV-curable resin monomer onto a substrate followed by exposing it to UV light while being kept in contact with a mold having a predetermined protrusion-groove microstructure profile, and the method is disclosed in, for example, Japanese Patent Publication No. JP-A 63-49702.
On the other hand, in optical communication, it is desired to prevent reflected and returning light in optical paths. In addition, it is necessary that the reflection on the surfaces of the optical devices is as small as possible. For such antireflection, a method of forming a multi-layered dielectric film on the surface of an optical device is the most popular.
However, in the resin forming technique mentioned above, the UV-curable monomer shrinks substantially in the step of photopolymerization thereof, and it could not often satisfy the designed dimensional accuracy that is necessary for optical devices. Another problem with the technique is that resin is not resistant to heat, and therefore, the temperature of the substrate on which a multi-layered dielectric film is formed must be kept low. Still another problem is that the quality and the durability of the multi-layered dielectric film are not so good.